1. Field of the Invention
The invention relates to a vehicular steering apparatus that includes a steering wheel that is operated by a driver, a front wheel steering mechanism that steers front wheels in response to an operation of the steering wheel, and a rear wheel steering mechanism that steers rear wheels in connection with the steering of the front wheels, as well as to a control method of that vehicular steering apparatus.
2. Description of the Related Art
In a four-wheel steered vehicle in which both the front wheels and rear wheels are able to be steered, the steering direction of the front and rear wheels must be pointed in the front-rear axial direction of the vehicle body in order to make the vehicle travel in a straight line while the front-rear axial direction of the vehicle body is aligned with the direction in which the vehicle is traveling. That is, when the operating position of the steering wheel that is operated by a driver is the neutral position, the steering direction of the front and rear wheels must be pointed in the front-rear axial direction of the vehicle body corresponding to this neutral position of the steering wheel. Therefore, in order to have the vehicle travel in a straight line while the front-rear axial direction of the vehicle body is aligned with the direction in which the vehicle is traveling, it is necessary to appropriately determine whether the vehicle is traveling in a straight line, and align the steering direction of the front and rear wheels with the front-rear axial direction of the vehicle body based on this determination.
Regarding this determination of straight-line travel of a vehicle, Japanese Patent Application Publication No. 2006-103390 (JP-A-2006-103390), for example, describes technology in which it is determined that a vehicle is traveling in a straight line when three conditions continue to be satisfied for a predetermined period of time. The three conditions are i) that the amount of change in the steering angle detected by a steering angle sensor be within a predetermined small amount of change, ii) that the vehicle speed detected by a vehicle speed sensor be equal to or greater than a predetermined vehicle speed, and iii) that the axial force applied to a rack bar be within a predetermined small range.
Also, regarding this determination of straight-line travel of a vehicle, Japanese Patent Application Publication No. 2003-276635 (JP-A-2003-276635), for example, describes technology in which it is determined that a vehicle is traveling in a straight line if i) the wheel speed of each wheel is greater than zero, ii) the wheel speeds of the left and right wheels are substantially the same, and iii) the steering torque is substantially zero.
Incidentally, in a four-wheel steered vehicle, even if the rear wheels are being steered slightly, the vehicle can still be driven in a straight line while the front-rear axial direction of the vehicle body is at a slight angle with respect to the direction in which the vehicle is traveling, by the driver operating the steering wheel and steering the front wheels in accordance with the rear wheels. However, in order to drive the vehicle, in a straight line in this way, the steering wheel must be held in an operating position that is different from the neutral position, i.e., the steering wheel must be held off-center. As a result, the driver feels the odd sensation that the neutral position of the steering wheel is offset even though the vehicle is traveling in a straight line. Therefore, it is necessary to correct the offset in the neutral position of the steering wheel that may occur in a four-wheel steered vehicle, i.e., it is necessary to correct the off-centerness of the steering wheel.